• Calculate probabilities and expected counts for a categorical variable

Italian Football (or in America, soccer)

In Italy, youth football leagues create cohorts of children based on year of birth.

For example, children born in 2015 only played with other children born in that same year. If a child was born on December 31, 2014, they played with the 2014 cohort (rather than the younger 2015 cohort). So, children born earlier in the year (e.g., January or February) tend to be the eldest players in their leagues. Children born later in the year (e.g., November or December) tend to be the youngest players in their leagues.

Could this seemingly unimportant practice—grouping by year of birth—have an effect on players’ later football careers?

In reality, birth rates are not the same between quarters of the year. Some quarters are longer (have more days), and different cultures have different preferences for times of birth.

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